This invention relates to an optical transmitter and optical transmission system. More particularly, the invention relates to an optical transmitter and optical transmission system having a function which compensates for wavelength dispersion in order to realize larger capacity, higher speed and longer transmission distance.
Though 10-Gbps optical transmission systems are currently in practical use, there is growing demand for networks of greater capacity owing to a marked increase in utilization of networks in recent years. In particular, highly precise dispersion compensation is required at transmission speeds above 10 Gbps. This means that measuring the dispersion value of an optical transmission line accurately and then compensating for this value is essential. The present invention relates to monitoring of the dispersion value of an optical transmission line and to a technique for optimum compensation of dispersion.
Signal energy contains a variety of components (different frequency components and different mode components), and signal waveform distortion will occur at the receiving end unless signal propagation delay time is constant. The phenomenon which is the cause of this delay distortion is referred to as xe2x80x9cdispersionxe2x80x9d and decides the transmission capacity of an optical fiber. In order to realize a further increase in capacity, speed and transmission distance in an optical transmission system, there is need for a technique for measuring dispersion with high precision and for compensating for this dispersion.
In a 40-Gbps optical transmission system, wavelength dispersion is one factor that limits transmission distance. Dispersion tolerance declines at the square of the bit rate, At 40 Gbps, dispersion tolerance is 30 ps/nm, which is much lower in comparison with a dispersion tolerance of about 800 ps/nm at 10 Gbps. FIG. 74A illustrates the relationship between amount of dispersion compensation and power penalty based upon a transmission experiment using 40-Gbps OTDM (optical time division multiplexing), 1.3-xcexcm zero-dispersion SMF (Single-Mode Fiber) having a length of 50 km. FIG. 74B shows the measurement system. The system shown in FIG. 74B includes an optical transmitter TX, a receiver RX, 1.3-xcexcm zero-dispersion SMF having a length of 50 km, and xe2x88x92920-ps/nm fixed-dispersion compensated fiber CB.
Though 1.3-xcexcm zero-dispersion SMF has been used for optical transmission lines, dispersion at high transmission speeds imposes limitations. In recent years, therefore, dispersion-shifted optical fiber for the purpose of reducing dispersion by shifting the zero-dispersion wavelength from 1.3 to 1.55 xcexcm has been developed and laid. The wavelength of light output from the transmitter is 1.55 xcexcm. As a consequence, 1.55-xcexcm light is transmitted via the 1.3-xcexcm zero-dispersion SMF laid originally.
When a 40-Gbps baseband signal is transmitted by a 1.55-xcexcm optical signal via a 1.3-xcexcm zero-dispersion SMF having a length of 50 km, dispersion of 920 ps/nm is produced. Accordingly, if 100% dispersion compensation is applied using the xe2x88x92920-ps/nm fixed-dispersion compensated fiber CB, reception sensitivity degradation will be 0 dB. However, if the amount of dispersion compensation is much larger or smaller than 100% dispersion compensation (=xe2x88x92920 ps/nm), reception sensitivity degradation rises and becomes 1 dB at amounts of dispersion compensation of xe2x88x92905 ps/nm and xe2x88x92935 ps/nm, as shown in FIG. 74A. In other words, if further dispersion in excess of xc2x115 ps/nm occurs at 100% dispersion compensation (amount of dispersion compensation=xe2x88x92920 ps/nm), reception sensitivity degradation will exceed 1 dB.
Accordingly, dispersion compensation tolerance when a power penalty of less than 1 dB is adopted as a condition for enabling transmission is a low 30 ps/nm, meaning that precise dispersion compensation must be carried out. Further, owing to temperature and stress which acts upon the optical fiber, the amount of change in transmission-line dispersion must be measured and the amount of dispersion compensation must be optimized within this narrow tolerance in conformity with change with the passage of time. Tolerance deviation DT due to temperature is as follows assuming a transmission line of SMF having a length of 50 km and a temperature change of xe2x88x9250xc2x0 C. to +100xc2x0 C.:
DT=0.03 (nm/xc2x0 C.)xc3x97150 (xc2x0 C.)xc3x970.07 (ps/nm2/km)xc3x9750 (km)=15.8 (ps/nm)
Thus there is the danger that the dispersion compensation tolerance of 30 ps/nm will not be met.
FIG. 75 is a characteristic diagram of wavelength dispersion, in which the wavelength (nm) of light output from an optical transmitter is plotted along the horizontal axis and amount of wavelength dispersion is plotted along the vertical axis. When a 40-Gbps baseband signal is transmitted by a 1.55-xcexcm optical signal via a 1.3-xcexcm zero-dispersion SMF, dispersion of 920 ps/nm is produced, as mentioned above. Accordingly, if 100% dispersion compensation is applied using the xe2x88x92920-ps/nm fixed-dispersion compensated fiber CB, wavelength dispersion becomes zero at 1.552 xcexcm. The zero-dispersion wavelength is 1.552 xcexcm (=1552 nm). If the wavelength of light output by the optical transmitter deviates from the zero-dispersion wavelength, wavelength dispersion of an amount indicated by the straight line in FIG. 75 is produced.
A method using 40-GHz component intensity in the baseband spectrum of an OTDM signal and NRZ signal has been considered as a method of wavelength dispersion compensation. This method utilizes a characteristic in which the amount of dispersion becomes zero and the eye pattern openness is maximized at a minimum point between two peaks of the 40-GHz component intensity.
FIGS. 76A and 76B illustrate the results of simulations of 40-GHz component intensity and eye openness with respect to amount of dispersion in case of a 40-Gbps NRZ signal, in which dispersion value (ps/nm) is plotted along the horizontal axis and 40-Gbps component intensity and eye openness are plotted along the vertical axis. FIG. 76A is for a case where xcex1 greater than 0 holds and FIG. 76B for a case where xcex1 less than 0 holds, where xcex1 is a chirp parameter representing direction (positive- or negative-going) and amount of fluctuation in a transmission waveform. Wavelength fluctuation (chirp) occurs when the voltage applied to an optical modulator increases or decreases owing to a rise and fall in a data pulse. Owing to the effects of chirping, (1) the rising edge of a pulse on the receiving side is delayed and the falling edge of the pulse is advanced (xcex1 less than 0), or (2) the rising edge of a pulse on the receiving side is advanced and the falling edge of the pulse is delayed (xcex1 greater than 0). The diameter of the eye opening is reduced by being shrunk along the time axis in the case of the former and is reduced by being stretched along the time axis in the case of the latter.
In accordance with FIGS. 76A and 76B, when xcex1=+0.7, xe2x88x920.7 holds, the 40-GHz component intensity peaks where the value of dispersion is in the vicinity of xe2x88x9240 ps/nm and +40 ps/nm, respectively, and the minimum value is obtained at the foot of the peak. Here the dispersion value is zero and the eye openness is maximum. The reason why the 40-GHz component intensity becomes zero when the dispersion value is zero (zero-dispersion wavelength=1552 xcexcm) and the eye openness is maximum is that in the case of the NRZ signal, 40 Gbps corresponds to 20 GHz and no 40-GHz component is included. This means that the zero-dispersion wavelength can be detected by detecting the foot of the 40-GHz component intensity.
FIGS. 77A and 77B illustrate the temperature characteristic (experimental values) of wavelength vs. 40-GHz component intensity in the case of the 40-Gbps NRZ signal. These are the results of transmission experiments at temperatures of xe2x88x9235 to +65xc2x0 C. using a DSF having a length of 100 km, in which FIG. 77A is for a case where xcex1 greater than 0 holds and FIG. 77B for a case where xcex1 less than 0 holds. As in the simulation results of FIGS. 76A and 76B, the minimum-value point at the foot of the peak of the 40-GHz component intensity in each Figure indicates the zero-dispersion wavelength (amount of dispersion=0) at plus and minus values of xcex1. It will be understood that the zero-dispersion wavelength changes while following up a fluctuation in temperature. More specifically, when the amount of dispersion of the optical transmission line fluctuates owing to a change in temperature, the zero-dispersion wavelength of the optical transmission line increases or decreases correspondingly.
Accordingly, it will suffice to detect the zero-dispersion wavelength and make the wavelength of light output to the optical transmission line equal to this zero-dispersion wavelength on the sending side. However, since the zero-dispersion wavelength must be detected, it is necessary for the wavelength of light to be varied continuously over a wide range. The state of the art is such that a tunable laser in which wavelength can be varied continuously over a wide range is difficult or impossible to realize because of structural complications and for reasons of cost.
Utilizing a semiconductor array laser (in which a number of laser-diode chips are formed on a single wafer) currently undergoing research or a plurality of discrete semiconductor lasers used in existing optical systems is believed to be closer to actualization. However, in the case of a semiconductor array laser obtained by forming a number of laser-diode chips of different wavelengths on one wafer or a plurality of discrete semiconductor lasers having different wavelengths, light is interrupted when wavelength is switched and, as a result, interruption of the transmitted signal occurs. In addition, a large difference in signal delay time before and after wavelength switching is produced and signal degradation occurs.
Accordingly, an object of the present invention is to compensate for wavelength dispersion in an optical transmission line by outputting, to this optical transmission line, light having a wavelength for which the transmission characteristic is optimum for this wavelength dispersion, this being accomplished even if a tunable laser is not used.
Another object of the present invention is to so arrange it that neither signal interruption nor signal degradation occurs at the time of wavelength changeover even if a plurality of light sources having different wavelengths are used, as when use is made of a semiconductor array laser or a plurality of discrete semiconductor lasers.
A further object of the present invention is to so arrange it that wavelength can be switched in a simple manner using an arrayed waveguide grating (AWG), a star coupler, a variable-wavelength filter or a light attenuator.
Still another object of the present invention is to so arrange it that dispersion compensation can be carried out accurately by detecting zero-dispersion wavelength even in a case where a plurality of light sources having different wavelengths are used.
Still another object of the present invention is to so arrange it that signal degradation is prevented by eliminating or reducing signal delay before and after wavelength changeover even in a case where a plurality of light sources having different wavelengths are used.
Still another object of the present invention is to compensate for wavelength dispersion in an optical transmission line by multiplexing monitoring light with main-signal light, detecting whether zero-dispersion wavelength has shifted in the direction of long wavelength or short wavelength and whether zero-dispersion wavelength has become an intermediate wavelength between wavelengths of neighboring light sources, and changing over light sources when zero-dispersion wavelength becomes an intermediate wavelength between wavelengths of neighboring light sources.
Yet another object of the present invention is to improve detection precision of, e.g., the direction of fluctuation of zero-dispersion wavelength by making the polarization of main-signal light orthogonal to the polarization of monitoring light in a case where monitoring light is multiplexed with main-signal light and then transmitted.
A further object of the present invention is to make it possible to support any optical modulation scheme, such as NRZ modulation, RZ modulation and OTDM modulation.
Yet another object of the present invention is to compensate for wavelength dispersion of an optical transmission line by multiplexing main-signal light and two monitoring light beams, in which the wavelengths of the main-signal light and monitoring light are different from each other, and changing over the main-signal light when the zero-dispersion wavelength fluctuates and the intensity ratio of two wavelengths between which the zero-dispersion wavelength is sandwiched attains a predetermined value.
Yet another object of the present invention is to so arrange it that wavelength dispersion compensation can be applied to a wavelength multiplexing optical transmission system.
In accordance with the present invention, the foregoing objects are attained by providing an optical transmitter having a wavelength dispersion compensating function, comprising: a plurality of light sources for outputting light having wavelengths that differ from one another; and means for outputting, to an optical transmission line, light of a wavelength whose transmission characteristic is optimum with regard to wavelength dispersion exhibited by this optical transmission line, the output light being obtained from light output by the plurality of light sources.
Further, in accordance with the present invention, the foregoing object is attained by providing an optical transmitter having a wavelength dispersion compensating function, comprising: a plurality of light sources for outputting light having wavelengths that differ from one another; means for outputting, to an optical transmission line, light of a wavelength whose transmission characteristic is optimum with regard to wavelength dispersion exhibited by this optical transmission line, the output light being obtained from light output by the plurality of light sources; and means for changing over the light generated by the light sources and outputting this light to the optical transmission line, thereby changing the wavelength of light output to the optical transmission line; wherein the wavelength of light output to the optical transmission line is changed, before start of operation of an optical transmission system, in order to detect a wavelength whose transmission characteristic is optimum with regard to the wavelength dispersion exhibited by this optical transmission line, and the light having the detected optimum wavelength is output to the optical transmission line during system operation.
Further, in accordance with the present invention, the foregoing object is attained by providing an optical transmitter having a wavelength dispersion compensating function, comprising: a plurality of light sources for outputting light having wavelengths that differ from one another; means for outputting, to an optical transmission line, light of a wavelength whose transmission characteristic is optimum with regard to wavelength dispersion exhibited by this optical transmission line, the output light being obtained from light output by the plurality of light sources; and means for causing fluctuation of the wavelength of the light output to the optical transmission line in order to detect a wavelength whose transmission characteristic is optimum with regard to wavelength dispersion exhibited by this optical transmission line; wherein the light having the detected optimum wavelength is output to the optical transmission line during operation of an optical transmission system.
Further, in accordance with the present invention, the foregoing object is attained by providing an optical transmitter having a wavelength dispersion compensating function, comprising: a plurality of light sources for outputting light having wavelengths that differ from one another; means for outputting, as main-signal light to an optical transmission line, light of a wavelength whose transmission characteristic is optimum with regard to wavelength dispersion exhibited by this optical transmission line, the output light being obtained from light output by the plurality of light sources; and means for multiplexing, with the main-signal light, monitoring light for detecting a wavelength whose transmission characteristic is optimum with regard to wavelength dispersion exhibited by this optical transmission line; wherein the monitoring light is multiplexed with the main-signal light during operation of an optical transmission system and the light having the detected optimum wavelength is output to the optical transmission line as the main-signal light.